[bugfix] avoid auxiliary keywords such as unquote or unquote-splicing

from being renamed

piclib/built-in.scm

@@ -599,6 +599,16 @@
     (let ((expr (walk (lambda (x) (if (symbol? x) (inject x) x)) expr)))
       (make-syntactic-closure mac-env '() (f expr inject compare)))))
 
+(define-syntax define-auxiliary-syntax
+  (ir-macro-transformer
+   (lambda (expr i c)
+     `(define-syntax ,(cadr expr)
+        (sc-macro-transformer
+         (lambda (expr env)
+           (error "invalid use of auxiliary syntax")))))))
+
+(define-auxiliary-syntax unquote)
+(define-auxiliary-syntax unquote-splicing)
 (define-syntax or
   (ir-macro-transformer
    (lambda (expr inject compare)